Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules. The Athabasca oil sand deposits may be efficiently extracted by surface mining which involves shovel-and-truck operations (for example, mining shovels and hydraulic excavators). The mined oil sand is trucked to crushing stations for size reduction, and fed into slurry preparation units (such as tumblers, rotary breakers, mix-boxes, wet crushing assemblies, or cyclofeeders) where hot water and, optionally, process chemicals such as caustic are added to form an oil sand slurry. The oil sand slurry may be further conditioned by transporting it using a hydrotransport pipeline to a primary separation vessel (PSV) where the conditioned slurry is allowed to separate under quiescent conditions for a prescribed retention period into a top layer of bitumen froth, a middle layer of middlings (i.e., warm water, fines, residual bitumen), and a bottom layer of coarse tailings (i.e., warm water, coarse solids, residual bitumen).
“Fines” are particles such as fine quartz and other heavy minerals, colloidal clay or silt generally having any dimension less than about 44 μm. “Coarse solids” are solids generally having any dimension greater than about 44 Bitumen froth is treated to produce diluted bitumen which is further processed to produce synthetic crude oil and other valuable commodities.
Oil sand extraction typically involves processing ores which are relatively high in bitumen content and low in fines content. However, there exists an abundance of “poor ores” which alone yield poor bitumen recovery and consequently cannot be processed unless a high proportion of high-grade, good ores are blended into these dry ore feeds, “Poor ores” are oil sand ores generally having low bitumen content (about 6 to about 10 wt %) and/or high fines content (greater than about 30 wt %). In comparison, “good ores” are oil sand ores generally having high bitumen content (about 10 to about 12 wt % or higher) and/or low fines content (less than about 20 wt %).
Blending dry oil sands is a common practice. Ore blending criteria include limiting the fines content in the ore feed to specified maximum levels to prevent processability problems, thereby limiting the maximum proportion of problem ores in the blends. Poor ores may be dry blended with good ores to achieve a feed fines content of less than about 28 wt %. As an example, ore blending criteria may include limiting the fines content to about 28 wt %, and/or the transition ore to a fines content of about 15 wt %, to ensure acceptable bitumen recovery.
However, it is not always possible to meet blending criteria, particularly for day-to-day operations. Blending is currently conducted by mining ore from separate locations in the pit, and transporting and feeding separate truckloads of ore into the slurry preparation and hydrotransport/conditioning system. The disadvantages of the current practice include a limited amount of ore that can be fed from each shovel, the necessity for shovel moves to maintain acceptable blends, and reduced throughput when processing large amounts of poor ores. Ore blending activities thus significantly increase operating costs and reduce production capacity. Accordingly, there is a need in the art for improved methods of extracting bitumen from poor ores.